The present invention relates to nano-sized stabilized zirconia and, in particular, to an economical method of making nano-sized stabilized zirconia.
Zirconia (ZrO2) exhibits toughness, wear resistance, hardness, low thermal conductivity and other properties that make it useful in numerous industrial applications.
Stabilized zirconia exhibits high fracture toughness, absorbs energy of impact that shatters other ceramics, and can tolerate thermal gradients better than most other high temperature materials. Because of its high oxyanion conductivity at elevated temperatures, stabilized zirconia is also the material of choice for electrolytes in solid-state fuel cells.
There are three commonly occurring and established crystal forms of zirconia: cubic, tetragonal, and monoclinic. The cubic form is the high temperature form and is stable above 2370° C. The tetragonal form is stable between 1170° C. and 2370° C. The monoclinic form is stable below 1170° C. The monoclinic to tetragonal phase change is accompanied by a volume change of about 4%. Cooling from the manufacturing temperature often destroys pure zirconia, or gives it inferior mechanical properties. Therefore, it is desirable to stabilize the zirconia in some fashion.
Several methods have been used to form stabilized zirconia. Generally, a stabilizing agent such as calcium, magnesium, yttrium, cerium, or rare earth oxides is added. Zirconia, structurally stabilized by the addition of calcium, yttrium, magnesium, cerium or rare earth oxides has numerous applications. One important application is the manufacture of oxygen sensors. It is also widely used today both for monolithic components and as a coating over refractory metal alloys to serve as a thermal barrier.
Recent advances have shown the advantages of manufacturing nano-sized stabilized zirconias. Nano-sized stabilized zirconia exhibits several very favorable properties, including significant reduction in sintering temperature and ability to deform superplastically under applied stress. Other enhancements include higher diffusivities and possibly higher ionic conductivities. These advantages play heavily both in the manufacture of solid oxide fuel cells and in producing spray coatings with superior mechanical attributes.
Existing methods to produce nano-sized stabilized zirconium oxide powders include co-precipitation and sol-gel synthesis. Existing gas-phase methods to produce nano-sized stabilized ZrO2 include inert gas condensation and chemical vapor condensation. These methods, however, are not economical to produce the bulk quantities needed for applications such as thermal barrier coatings and solid oxide fuel cells.
U.S. Pat. No. 6,162,530 discloses a process to make nano-sized powders of ZrO2 from aqueous solutions. The disclosed process involves atomizing an aqueous solution of the desired metals in a stream of nitrogen and contacting the resulting particles with a spray of a recirculating aqueous solution at controlled pH. The particles produced by this method consist of nanostructured fibers. Further treatment includes sequential heat treatment, ultrasonication, and spray drying.
Although this process may be simpler and cheaper than the co-precipitation or sol-gel synthesis, the process still requires steps that become expensive and difficult to control when extrapolated to commercial production scale, e.g., the efficient mixing of the atomized particles with the recirculating solution requires a special reactor and filtration of the particles will in practice require cumbersome equipment and produce large streams of waste solutions.
The process of the present invention does not produce fibers but particles of approximately the same size in all three dimensions, and solves the above problems by providing an economical and commercially practical method for making stabilized zirconia.